A standard rock drill has a main base that is carried on a positioning arm and that in turn carries a support that can move on this base in a predetermined longitudinal direction, with a main ram or actuator braced between the two parts for moving the support on the base longitudinally. The drill in turn can slide or roll longitudinally on this support with another actuator braced between the drill and the support to control its longitudinal movement. The drill carries a tool or bit that can be rotated, and often also reciprocated longitudinally slightly, for drilling into the rock face.
The standard procedure in such prior-art structures is to pressurize the main actuator to press the support against the wall with a predetermined very large anchoring force, whereupon the drill bit is brought to bear to form the hole. The drill itself is urged against the wall with a drilling force that is less than this large force and normally as great as the drilling bit can withstand. The two biasing forces are generally set by simple pressure-limiting valves connected between the respective actuators and a source of pressurized hydraulic liquid.
This system has several disadvantages:
The support can slip on the surface as the drill is being set up.
The rock face spalls where it is engaged by the support due to the considerable pressure exerted against it, particularly during drilling.
The drill can move during drilling.
As the drill bit encounters harder and softer material the anchoring force varies, thereby excessively stressing the rock face where the support engages it.
When the rock face is not perpendicular to the longitudinal drilling direction, which is invariably parallel to the direction of application of the anchoring force, it is very difficult to solidly position the support on the inclined face.
Drilling is generally inaccurate.